Metal halide lamps are known in the art and are described in, for instance, EP215524, EP587238, WO05/088675 and WO06/046175. Such lamps operate under high pressure and comprise ionizable gas fillings of, for instance, NaI (sodium iodide), T1I (thallium iodide), CaI2 (calcium iodide) and REI3. REI3 refers to rare-earth iodides. Characteristic rare-earth iodides for metal halide lamps are CeI3, PrI3, NdI3, DyI3 and LuI3 (cerium, praseodymium, neodymium, dysprosium and lutetium iodide, respectively).
There is a continuous effort in industry to optimize such lamps and their production process. Lifetime and energy-saving aspects of the lamps as well as reduction of costs involved in the production process of the lamp are items that are investigated.
One specific item of interest is the lifetime of the lamp. Substantially long lifetimes are desired, without, however, a substantial change of lamp characteristics.
Another item of interest is, for instance, the reduction of costs during the production process. For instance, lowering the heating temperature during a sealing step in the production process might be of interest in view of saving costs. In the present production process of metal halide lamps, the lamps are sealed at relatively high temperatures. A reduction of heating time and/or heating temperature would be beneficial for the apparatus used for performing such a sealing step, but might also be beneficial for the lifetime of the lamp (less risk of crack formation).
A further specific item of interest is matching the thermal coefficient of expansion of the material of the seal with the material of the current lead-through conductors and/or the material of the discharge vessel. In general, the better the match, the longer the lifetime and/or the less risk of defective lamps in modern lamp production processes of large quantities on an industrial scale. A better match will also reduce the risk of crack formation.
Yet another item of interest is the possibility that the filling constituents (such as mentioned above) within the discharge vessel react with the sealing material and/or that elements in the sealing material have an impact on the filling constituents in the discharge vessel, which processes may have a negative effect on lamp lifetime and/or stability of lamp characteristics.